1. Field of the Invention
The present invention relates to an arrangement and process for bonding a piezoactuator and a printed circuit board. More specifically, the present invention relates to an arrangement and process for bonding a piezoactuator and a printed circuit board utilizing an electrically conductive adhesive between a first contact on the piezoactuator and a second contact on the printed circuit board with enhanced reliability and security.
2. Description of the Related Art
Arrangements are known consisting of a piezoactuator and a printed circuit board and are used, for example, in so-called vibration boundary state measuring apparatuses. The schematic construction of such a vibration boundary state measuring apparatus is shown in FIG. 1; and, is neither true to scale nor complete and is intended solely to show the function of a vibration boundary state measuring apparatus in the manner of a diagrammatic sketch.
The vibration boundary state measuring apparatus shown comprises an oscillating fork 11 as measuring sensor which fork is arranged on a membrane 9. Membrane 9 is part of oscillating fork 11, whereby prongs 11a, 11b of the oscillating fork are directly bound on membrane 9 and are move to and from one another by bending membrane 9.
Membrane 9 is manufactured as a rule from high-grade steel and can be put in oscillation by a piezoactuator. Piezoactuator 1 is connected via a ceramic platelet 7 arranged between piezoactuator 1 and membrane 9 to the latter. Ceramic platelet 7 is provided in order to adapt the different thermal coefficients of expansion of membrane 9 and of piezoactuator 1 to each other.
An exciting of piezoactuator 1 takes place via an electrical voltage that is supplied via a printed circuit board 3 on the top of piezoactuator 1. A bottom of piezoactuator 1 is continuously metallized so that a voltage applied to the surface brings about an oscillation of piezoactuator 1 on account of a segmentation of the electrode on the top side as well as on account of a different polarization of these piezosegments. Printed circuit board 3 is constructed in the present example as a so-called flexline, i.e., as a flexibly constructed printed circuit board with conducting tracks arranged on it in a manner known to those of skill in the art.
Flexline 3 is connected at the points to be contacted of piezoactuator 1 via conductive adhesive to the latter so that a voltage is introduced into piezoactuator 1 exclusively at the points connected by the adhesive. Piezoactuator 1 as well as ceramic platelet 7 and membrane 9 are also adhered to each other.
In order to produce a vibration of oscillating fork 11 with piezoactuator 1 and at the same time to detect a frequency shift or damping of oscillating fork 11, piezoactuator 1 is divided into four segments, not shown but as will be understood by those of skill in the art. Piezoactuator 1 is constructed as a rule in a circular manner in a top view and subdivided into four quadrants that can be individually regulated by applying a voltage. In order to produce an oscillation, two diagonally opposite quadrants are loaded with a voltage while the other two quadrants are used to detect the produced oscillation. In this manner, an oscillation can be simultaneously produced and detected.
FIG. 2 shows the arrangement consisting of piezoactuator 1 and printed circuit board as it is customary in the state of the art. Printed circuit board 3 (or the “flexline”) has a first connection contact 13 for applying the voltage on piezoactuator 1. A second connection contact 15 is provided on piezoactuator 1 corresponding to the first connection contact 13. In order to produce an electrical as well as a mechanical connection between flexline 3 and piezoactuator 1, it is customary according to the state of the art to produce an electrically conductive adhesive connection 5 between the first connection contact and the second connection contact. It is currently customary in the state of the art to first apply the electrically conductive adhesive on connection contacts 15 of piezoactuator 1 and to subsequently arrange flexline 3, suitably aligned, on the latter and to adhere it to piezoactuator 1.
This conventional practice has several detriments that have no resolution. Due to the small adhesive surface and the subsequent pressing, an amount of adhesive must be very small and dimensioned very precisely. An exactly reproducible dosing of a very small amount of a highly viscous medium can only be achieved with difficulty due to the surface forces dominating in small volumes, so that there is an elevated production cost and a detrimental related defect rate.
This procedure has the further operative problem that a false alignment of flexline 3 in the first attempt can no longer be corrected since otherwise the conductive adhesive is falsely smeared between connection contacts 13, 15 and as a consequence short circuits between individual contacts must be reckoned with. It is furthermore a detriment to operability that flexline 3 and piezoactuator 1 cannot be arranged directly on top of one another on account of adhesive connection 5 between them. This arrangement creates necessarily a space or gap (shown in FIG. 2) which further is an operative detriment. It is moreover negative that when flexline 3 and piezoactuator 1 are pressed together, adhesive connection 5, located between them, is pressed flat and as a result any excessive adhesive is pressed out laterally next to connection contacts 13, 15. This, in turn, increases the risk and the danger of short circuits.
As a further operative concern, on account of the purely material connection between flexline 3 and piezoactuator 1 in FIG. 2 every later loading of adhesive connection 5 must be taken up by forces of cohesion and adhesion.
What is not appreciated by the prior art is that after the application of the adhesive, the flexline 3 must be positioned plane parallel and exactly to piezoactuator 1. For the reasons presented above, a subsequent correction of any mis-positioning is not possible and is a further operative detriment.
The present invention has the problem of eliminating the above-described disadvantages and of simplifying the carrying out of the process in the manufacture of the described arrangement.
Accordingly, there is a need for an improved arrangement and process for bonding a piezoactuator and a printed circuit board utilizing an electrically conductive adhesive between a first contact on the piezoactuator and a second contact on the printed circuit board.